1. Field of the Invention
This invention relates to a single step process for making beryllium oxide from the hydroxide form.
2. State of the Art
Beryllium oxide, also known as beryllia, is an important amphoteric oxide. It reacts with acids to form salts and with alkalis to form compounds known as beryllates. Beryllium oxide is used in the production of beryllium and beryllium-based compounds such as beryllium-copper alloys. End-use applications for BeO are directed to transistors, electron tubes, semiconductor packages, resistor cores, organic catalysts, windows for klystron tubes and high temperature reactor systems. BeO is a unique ceramic material. It has beneficial mechanical, thermal and electrical properties. The BeO powder can be fabricated into finished shapes which are transparent to microwave and gamma radiation.
Beryllium oxide is conventionally produced from beryllium hydroxide. But, until now, art-recognized techniques have provided BeO powders containing elevated levels of fluorine or other impurities which are unacceptable for high density beryllium oxide ceramics. Commercial practices for recovery of beryllium hydroxide (Be(OH).sub.2) from beryl ore are described in The Metal Beryllium, White and Burke, American Society For Metals (1955). Gadeau U.S. Pat. No. 1,925,920 discloses a modification of the art-recognized Copaux process. It describes beryllium hydroxide preparation from beryl ore which is reacted with sodium fluorosilicate at 650.degree.-700.degree. C. The reaction mass is water leached to solubilize beryllium as a solution of sodium fluoroberyllate. Dry sodium fluoroberyllate is mixed with sodium carbonate and recalcined between 525.degree. and 550.degree. C. The second calcination product is water leached to solubilize sodium fluoride. Beryllium oxide is left as a wet paste after solid-liquid separation and several water washings of the solid. This wet paste beryllium oxide product retains elevated levels of fluoride or sodium which are unsuitable for preparing high density beryllium oxide ceramics.
H. C. Kawecki, "The Fluoride Extraction of Beryllium from Beryl," Chapter IV, Part B, The Metal Beryllium (1955) and U.S. Pat. No. 2,312,297 describe a modification of the Copaux process wherein beryllium is liberated from the ore by interaction of sodium ferric fluoride and sodium fluorosilicate at 750.degree. C. A water leach of the sintered reaction mass solubilizes the beryllium in sodium fluoroberyllate. Beryllium hydroxide is recovered by adding excess sodium hydroxide (50 weight percent caustic) to the fluoroberyllate leach solution. A concentration of 20 percent by volume is achieved. The remainder of the fluoroberyllate leach solution is then added slowly to form Be(OH)2 which is filtered as a granular precipitate from the warm solution.
Beryllium hydroxide from the Kawecki-Copaux process is similarly limited by the presence of impurities. The resulting product contains sodium fluoride which makes it unsuitable for direct production of ceramic-grade beryllium oxide. A further modification of this process is described in Kida U.S. Pat. No. 3,233,970. Like other prior art processes, the Kida modification produces beryllium hydroxide containing sufficient sodium fluoride or ammonium fluoride which makes it unsuitable for ceramic-grade beryllium oxide.
Bhat and Moorthy, "Influence of Minor Additions on Sintering Behaviour of BeO," Trans. Ind. Ceram. Soc., Vol. 29 (3), pp. 72-78 (1970) describe the precipitation of gelatinous beryllium hydroxide by the neutralization of beryllium sulfate solution with ammonium hydroxide. A companion paper entitled "Influence of Minor Additions and Calcination Temperature of Beryllia on Powder Properties," Trans. Ind. Ceram. Soc., Vol. 40 (5), pp. 161-165 and 174 (1981) discloses the addition of magnesium succinate, titanium sulfate or ferric ammonium sulfate to the beryllium sulfate solution prior to ammoniacal precipitation.
These processes typically provide impure beryllium oxide which is not suitable for advanced ceramic applications. Ammonium sulfate is a principal contaminant produced by the Bhat and Moorthy processes. Gelatinous beryllium hydroxide products formed by alkali neutralization reactions are similarly contaminated with the companion neutralization salt, generally sodium sulfate or ammonium sulfate.
Another commercial process for producing beryllium oxide is a two step process requiring the conversion of beryllium hydroxide to beryllium sulfate tetrahydrate and subsequent calcination of the hydrated sulfate to the oxide at 1100.degree. to 1300.degree. C. The latter reaction step produces SO.sub.2, an environmentally damaging gas which requires further processing to convert the acidic off-gas to a non-hazardous form for disposal.
The previously described process can be modified by using an additive selected from urea, ammonium oxalate, ammonium acetate, and ammonium sulfate. These modifications are described in Schwenzfeier U.S. Pat. No. 3,172,728. Additives are chosen for their ability to completely vaporize during calcination, without the formation of contaminating residues. But, the use of an intermediate composition in a multi-step process is undesirable.
Brush Wellman Inc. (previously known as Brush Beryllium Corp.) used a fusion-quench process developed by Sawyer and Kjellgren to liberate the beryllium from beryl ore. This process is disclosed in U.S. Pat. No. 1,823,864. C. W. Schwenzfeier, Jr., "The Sulfate Extraction of Beryllium from Beryl," Chapter IV, Part C, The Metal Beryllium (1955) describes the commercial practice of this fusion-quench process and subsequent solubilization of beryllium with sulfuric acid. After treatment of the beryllium sulfate leach solution to reject aluminum, the solution is made basic by adding excess sodium hydroxide. Beryllium, which first precipitates as amorphous beryllium hydroxide, is redissolved in excess caustic as a sodium beryllate complex (Na.sub.2 BeO.sub.2). The solution is filtered, carefully diluted and boiled to precipitate a granular beryllium hydroxide which is recovered by additional filtration.
The original Sawyer-Kjellgren process was subsequently modified by applying solvent extraction technology to the sulfate leach solution. Maddox U.S. Pat. No. 3,259,456 describes the recovery of beryllium from the organic extraction solvent by separation with aqueous ammonium carbonate. The resultant ammonium beryllium carbonate solution is purified and boiled to precipitate beryllium basic carbonate. This beryllium basic carbonate is normally converted to beryllium hydroxide by a pressure hydrolysis step at 165.degree. C. Granular beryllium hydroxide is recovered by filtration.
Some modern beryllium hydroxide production is still based on beryl ore input. But, most commercial production is now derived from sulfuric acid leach of bertrandite ore. Beryl leach solutions can be mixed with bertrandite leach liquors prior to solvent extraction and generation of the ammonium beryllium carbonate solution. Beryllium hydroxide is formed by pressure hydrolysis of the ammonium beryllium carbonate solution. It is filtered, lightly washed and drummed as a damp granular powder containing approximately 20 percent free moisture.
Improved purity for Be(OH).sub.2 is achieved by the Maddox solvent extraction process. The beryllium hydroxide is relatively free of cationic impurities, but residual fluoride (1-2 weight percent) has prevented its direct calcination to an acceptable ceramic-grade beryllium oxide. BeO is commercially produced by a two-step process which requires the conversion of beryllium hydroxide to beryllium sulfate tetrahydrate, and subsequent calcination of the sulfate to the oxide at 1100.degree. to 1300.degree. C.